Reset SMTP auth when setting SMTP auth type

This change ensures that the smtpAuth field is reset to nil whenever the SMTP auth type is updated. This prevents potential issues with mismatched authentication settings.

client.go

@@ -578,6 +578,7 @@ func (c *Client) SetPassword(password string) {
 // SetSMTPAuth overrides the current SMTP AUTH type setting with the given value
 func (c *Client) SetSMTPAuth(authtype SMTPAuthType) {
 	c.smtpAuthType = authtype
+	c.smtpAuth = nil
 }
 
 // SetSMTPAuthCustom overrides the current SMTP AUTH setting with the given custom smtp.Auth